Valve proof check credit score for a course of trip

A course of trip happens when the safety instrumented system (SIS) locations the process in its protected state by commanding an automatic valve (Figure 1) or other tools to its journey state in response to an irregular process situation. In some circumstances, a spurious journey happens due to a valve malfunction. In that case, the valve assembly is tested beneath actual working situations, which offers a chance to capture valuable valve diagnostic knowledge.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs may help decide the proof test credit score for an automatic valve after a process trip.
Process trip
A course of journey occurs when the SIS detects an abnormal course of situation through sensors similar to temperature and pressure, executes the logic and places the process in its protected state by tripping the ultimate elements similar to closing an automatic valve, stopping a pump, and so forth. The SIS might communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a course of journey happens, the primary objective is usually to restart the unit or equipment that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken units. Taking เกจ์ลมsumo to proof check an automated valve will not be a top precedence and even an exercise under consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof check of the valve can be considered carried out with diagnostic data captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline might help uncover valve deficiencies and degradations that will not show up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller during a process journey meet most of the 12 requirements of a proof take a look at.
Process trips versus proof tests
How can proof check credit be claimed for an automatic valve that has failed and triggered a course of trip? A proof take a look at is a periodic check performed on SIS sensors, logic solvers and ultimate parts — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, could impair the ability of the SIS to take the process to its safe state when an abnormal course of condition is detected.
A proof take a look at ought to be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually decided via a mean chance of failure on demand calculation (PFDavg). Some customers might select to proof test based mostly on predetermined intervals, similar to proof testing sensors every 24 months and last elements every forty eight months as an alternative of counting on the PFDavg calculations.
Automated valve proof tests can be done offline or online. Offline proof checks are usually scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve on-line often requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and costly.
But a proof test can be achieved during a course of trip.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns due to actual demand on the SIS throughout operation may be given credit score as proof checks (fully or partial) under given conditions…the subsequent planned proof check could also be skipped.”
These conditions are
The shutdown documents equivalent information as registered throughout corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the system or SIS subsystem not activated must be tested separately.
The shutdown happens inside a predetermined most time window earlier than the following planned proof take a look at which may then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought of carried out. A sample record of activities carried out throughout a proof test, along with those that are performed during a course of trip, is shown in Figure 2. Even with out an automatic valve leak test, data captured by the DVC alone can probably account for an excellent amount of proof take a look at coverage for an automated valve.
The exact coverage depends on the design of the automated valve, its DVC and its software. The protection is estimated based on the valve degradations, the chance of their occurrence and the percentage of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can often be enough to fulfill a serious part of the proof check requirements.
If the process journey takes place inside a predetermined maximum time window, the tip person could select to leverage the process journey as a proof test by completing steps one by way of five in Figure 2, which are usually not accomplished in a process journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window should be the final half of the current proof test interval.
Figure 3. Data during a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there for evaluation.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most likely root causes of automated valve degradations — which may lead to a course of trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, pace controller, and so forth., as a result of moisture, debris or alignment points. This causes a loss of useful margin and makes the valve slow to open or shut.
Binding, galling or other degradation of valve seats or related circulate management trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, put on or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball brought on by system conditions, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system parts similar to solenoids, pilots, pace controller, and so on., because of moisture, debris or alignment. The valve will fail to open or close.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s caught open will exhibit itself as an increase in breakout force. Loss of seat load in comparison with when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly monitors for inside faults as nicely as its inputs such as supply pressure. When the DVC6200SIS sees the availability pressure is just too low, an alert is generated (Figure 4) so that the end person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide stress, so it alerted the control system.
Other course of journey advantages
Diagnostic information captured during a course of trip could reveal valve degradations that is most likely not detected during a proof take a look at. For instance, diagnostic knowledge captured throughout a process trip would possibly point out an issue with the valve closing fully in opposition to the total strain of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of journey is extra accurate under actual working situations. This leads to a more correct SIF response time (the time from detection of an abnormal condition to final element reaching its trip state), which is compared to the Process Safety Time to make sure the SIF is still assembly its design necessities.
Although automated valves are very dependable, failures identified in a process trip can present valuable data to stop future failures. This info can help with turnaround planning by making sure the needed components are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of trip can present the protection required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC may be analyzed to leverage the method trip as a proof take a look at. Even if the tip consumer chooses not to take proof test credit for a process trip, the valve diagnostic knowledge supplied by the DVC can help plant personnel make proactive valve upkeep selections..
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